Standards for evaluating the structural integrity of vessels whose contents are to be stored under pressure are adopted by the American Society of Mechanical Engineers, the American Society for Testing of Materials, and other professional and industrial organizations. Certain governmental agencies adopt and enforce such standards. One of those agencies is the United States Department of Transportation which has promulgated test procedures to be employed in testing cylinder used in transporting pressurized gasses. Those procedures are set out in the Department's "Hazardous Material Regulations" in 49 CFR, subpart B, 107 et seq. The regulations include a requirement that certain classes of cylinders by hydrostatically retested periodically. Cylinders are immersed in a water tank and are pressurized to a value above service pressure. The vessel's volumetric increase incident to pressure increase is measured, and structural integrity is assumed if the vessel has not ruptured and if volumetric increase has not exceeded a predefined proportion of its unpressurized volume.
The Department of Transportation tests determine the likelihood of eminent rupture and any reduction in wall thickness resulting from corrosion. The number of pressure vessel failures is very small, which suggests that the procedure is basically sound from a safety standpoint. On the other hand, the good safety record may indicate that safety margins in vessel design are more than adequate. The combination of better test methods and narrower safety margins may result in cost saving both in the initial production and in the handling and transportation of vessels, and in increased safety. A substantial amount of effort is being devoted to the development of improved testing methods and apparatus in the search for greater safety at lower cost.
The kind of fault that is not detected in the hydrostatic pressure testing is the growth of microscopic cracks that can, and do, become large cracks and eventually rupture points.
The formation of microscopic cracks in metals, including pressure vessel steels, is the mechanism by which internal stresses developed in the material in the material making or vessel fabrication processes are relieved. Subsequent stressing of the material, as by subjecting a vessel to internal pressure, causes relative movement of the material about the crack, and that movement generates acoustic events which can be detected using microphones. The frequencies of those events are in the ultrasonic range in general, and the microphones are called acoustic transducers. They rely on piezoelectric phenomenon, are very sensitive to a wide range of frequencies. They are high impedance devices which gives rise to considerable difficulty in practice. PG,4
Large scale users of pressure vessels subject to Department of Transportation test requirements, interested in minimizing the cost of periodic testing, have done a substantial amount of work in an effort to develop tests that can be conducted without immersion in water and without removing the vessels from their carriers. Acoustic events are initiate by pneumatic pressurization in open air and are detected. Interpretation of results is made very difficult because, in that environment, the signal to noise ratio is low. In practice, supplimentary non-destructive testing using penetrating dyes, magnetic flux and ultrasonic thickness measurements and the like are required. It is necessary to use multiple piezoelectric transducers and to measure differences in signal arrival times whereby to permit mapping of cracks. Acoustic events occur as bursts of acoustic energy, and are rich in harmonics. The result is a mass of acoustic data including phase differences in primary data, reflections, many frequencies, and noise both mechanical and electrical in a wide range of frequencies and energy levels.
To cope with that situation, it has been necessary to establish threshholds of amplitude, signal rise time, interval between events and the like, above which further inspection is assumed to be required and below which the test is assumed to have been passed. The threshholds are established at some value above worst condition ambient noise, which means above the aggregate of the acoustic value of conversion, air movement, temperature induced dimensional change, and vibrations caused by movement of people and things in the vicinity of the test. In the last analysis, the threshold is set at some arbitrary value which seems to permit accurate mapping of event origins.
The sensitivity of acoustic sensors is entirely adequate, and the use of preamplifiers as impedance changers greatly reduces the problems of electrical noise in processing of acoustic event signals. What is required, and what it is a purpose of this invention to provide, is test apparatus and techniques that permit better recognition of acoustic events that are the precursors of vessel failure.